Breakdown and Pitting Formation of Anodic Film Aluminum Alloy (3003)

Sami A. Ajeel, Nahidh W. Kasser, M.Sc. Basheer A. Abdul-Hussein

Abstract

Aluminum alloy (3003) has been anodized using sulfuric acid solution. To study the characteristic of the anodic film of aluminum alloy (3003); four variables are examined in term of anodic film thickness, these are current density ranging between 1.5- 3.5 A/dm2, electrolyte concentration ranging between 10- 20 Vol.%, electrolyte temperature between 10- 30oC and anodizing time between 10- 50 min.

The study shows that the time of anodizing and current density has positive dependence of great significance on the anodic film thickness of aluminum alloy (3003) while the other two studied variables (i.e. concentration and temperature of electrolyte) show little dependence on the film thickness.

When conditional Hookes and Jeeves optimization method is used, optimum conditions of aluminum alloy (3003) in terms of maximum thickness are found equal to:

Current density (A /dm2) 3.5

Acid concentration (Vol. %) 10

Electrolyte temperature (oC) 19

Time of anodizing (min.) 50

Aluminum alloy (3003) specimens at optimum conditions are anodized and comparison studies between anodized and un-anodized specimens are carried out in terms of:

1) the roughness and hardness of anodic film.

2) the corrosion rates in 3.5% NaCl solution by:

a) immersion test.

b) polarization curves.

3) examining microstructure before and after anodizing of both types and their corrosion specimens by optical microscope and X-ray diffraction.

In general, it is found that the surface roughness and hardness values for the anodized specimens are greater than that of un-anodized specimens because of the very much thicker oxide coating compare with the natural oxide (atmospheric oxide) whose improved physical and chemical properties. Also, the corrosion rates for anodized specimens are lower than that for un-anodized ones.

The polarization behavior for anodizing alloy shows that the breakdown potentials are shifted to more noble direction than bare metals due to anodizing.

The study also shows that the Breakdown of passive film and pitting formation occurred at (-661mV) for un-anodized alloy, while (-410 V) for anodized alloy. The observed pits are clearly not deep and small in size in large numbers compared with the unanodized specimens.

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